1. Field of the Invention
This invention relates to an air conditioning apparatus having a power-variable type compressor whose power is variable using an inverter, and particularly to an air conditioning apparatus using an improved adjustment technique for adjusting the output of a source voltage detection circuit.
2. Description of Related Art
Recently, an inverter air conditioning apparatus (hereinafter referred to as "inverter air conditioner") has broadly propagated as an air conditioning apparatus which is capable of adjusting the temperature in a room or the like in fine level. The inverter air conditioner is designed so that the rotating frequency of a motor of a compressor is varied in accordance with an air conditioning load which is detected on the basis of the temperature detected by a temperature sensor, thereby varying a compression power of the compressor. Through this air conditioning operation, a space such as a room or the like is kept in the optimum air conditional state.
In general, a package type inverter air conditioner adopts an open loop control operation using a V/f control in which the motor is controlled on the basis of the ratio (V/f) of an output voltage of the inverter (an input voltage of the motor) and a driving frequency f of the motor.
In the V/f control type inverter air conditioner as described above, when the input voltage of the inverter is varied (fluctuated), the output voltage of the inverter is also varied substantially at the same level as variation of the input voltage at the same driving frequency. In consideration of the variation of the input voltage to the inverter, a permissible voltage range .DELTA.V with a center voltage C at the center of the voltage range is set for the output voltage to avoid troubles due to the variation of the input voltage to some extent.
However, when the input voltage is beyond a variation upper-limit value UL or a variation lower-limit value LL, the following trouble occurs.
That is, when the input voltage increases greatly at a low driving frequency f1 represented by a in FIG. 7, the output voltage exceeds the upper limit value UL and thus an excessive voltage is applied to the motor, so that the motor falls into an overexcitation state. If the overexcitation is remarkable, the temperature of an excitation winding of the motor is increased, and the excitation winding may be burned out.
On the other hand, when the input voltage decreases greatly at a high driving frequency region f2 to f3 which is represented by b in FIG. 7, the output voltage of the inverter is beyond the lower limit value LL to induce torque lack to the motor, so that the driving efficiency of the inverter is lowered.
For inverter air conditioners for overseas, that is, inverters for exportation, they must be designed on the basis of voltage specifications which meet the respective power circumstances of import countries, however, it is impractical in cost to design individual types of inverter air conditioners to meet the voltage specifications for the power circumstances of the respective countries. Therefore, it is preferable that the inverter air conditioners are commonly designed to be of a wide range type in which an usable voltage range is wide. However, various different commercial source voltages are used in individual countries in the world, and thus it is extremely difficult to provide these countries with only one kind of inverter air conditioner even if it is designed in consideration of occurrence of the voltage variation (fluctuation) as described above. Particularly, the above problem due to the voltage variation becomes more remarkable in the wide range type of inverter air conditioner.
Further, in order to provide the wide range type of inverter air conditioner, it is required to automatically detect the source voltage (input voltage VIN) of a voltage source to which the inverter air conditioner is connected to perform the V/f control in accordance with the detected voltage VD.
In view of the foregoing, in this type of conventional inverter air conditioner, the source voltage VIN is detected using a source voltage detection circuit which comprises a stepdown transformer rectifying circuit and a smoothing circuit, and then a detection signal VD (DC voltage) representing the input source voltage VIN is supplied from the source voltage detection circuit to an A/D conversion input port of a microcomputer serving as a controller in the inverter air conditioner to perform the V/f control in accordance with the source voltage on the basis of a driving control program stored in the microcomputer.
With this construction, the source voltage detection circuit is designed so that the corresponding relation between the source voltage VIN and the detection signal VD is set to meet the conversion rate of the A/D conversion input port attached to the microcomputer. For example, for the source voltage VIN of AC 400 V, the source voltage detection circuit is designed so that the detection voltage (signal) VD is equal to 3.91 V.
However, some actually-manufactured source voltage detection circuits do not necessarily provide the detection voltage VD which is expected in circuit design because of fluctuation in characteristics of stepdown transformers, diodes of rectifying circuits or other resistance elements, etc.